This invention relates to a vehicle traction control system and method and specifically to such a system and method for preventing side-to-side spin cycling.
When the operator initiated torque delivered to the driven wheels of a vehicle during vehicle acceleration are such that the frictional forces between the tire and the road surface are overcome, excessive wheel spin results. While a small amount of spin between the tire and road surface is necessary in order to achieve a driving force, excessive spin results in the reduction of the effective driving force and in the deterioration in the lateral stability of the vehicle.
Various methods have been proposed for preventing an excessive spinning condition of the driven wheels of a vehicle by limiting the spin between the wheel and the road surface at a value that will achieve a maximum transfer of driving force from the driven wheels to the road surface. One such method senses the excessive spin condition at a driven wheel and applies the brakes of that wheel by an amount that limits the slip at a desired value.
Such traction control systems which use the left and right driven wheel brakes to independently reduce the velocity of the left side and right side spinning wheels to limit wheel spin may under certain conditions induce an out of phase or side to side spin cycling action. This condition may result in a deterioration in performance quality and may add stress to the vehicle powertrain components. In general, the condition arises as follows. The brake pressure applied to one of the spinning wheels will result in a reduction of that wheel velocity, but will also increase the level of the torque delivered to that half shaft. The torque on the half shaft is the sum of the torque delivered to the ground and the torque needed to brake the spinning wheel. Since the differential delivers equal torque to both drive wheels (via the half shafts), that increase in torque will also be delivered to the opposite drive wheel. The non spinning drive wheel starts to accelerate while the spinning drive wheel is decelerating. The increased torque to the non-spinning wheel is enough to cause this wheel to now spin. The original spinning wheel, since the spin has been reduced, does not require as much brake pressure as it originally had and now causes a reduction in torque. The second spinning wheel now requires an increase in brake pressure which produces an increase in torque. This situation, if out of phase, could cause the undesirable cyclic side to side spin action.